1. Field of the Invention
The present invention relates to a focus detection device, for a camera or the like, which employs electric charge accumulation type photoelectric conversion devices.
2. Description of Related Art
There is a per se known focus detection device which detects the focus adjustment state of a taking lens of a camera or the like by using electric charge accumulation type photoelectric conversion devices. This type of focus detection device comprises an optical system for focus detection, an auto focus sensor module (hereinafter termed an A/F sensor module) which incorporates a pair of A/F sensors, and a calculation device which incorporates a microcomputer. The optical system for focus detection directs onto the pair of A/F sensors a pair of luminous flux from the object to be photographed which have passed through a pair of regions symmetrical with respect to the optical axis of the exit pupil face of the taking lens, and forms an image of the object to be photographed on each of these A/F sensors. Each A/F sensor is made up from a plurality of electric charge accumulation type photoelectric conversion devices, and performs accumulation of electric charge according to the intensity of illumination of the object to be photographed in each of its photoelectric conversion devices for a previously determined time period, and then outputs electrical signals for focus detection corresponding to the distribution intensity of illumination of the image of the object to be photographed. These electrical signals for focus detection are converted into digital signals by an analog to digital converter (hereinafter termed an A/D converter), and then are processed by a predetermined algorithm in a calculation device, so that the defocusing amount of the taking lens is calculated and its state of focusing is obtained. This defocusing amount is defined as the distance between the plane on which an image of the object to be photographed is focused by the taking lens, and the film plane.
The longer is the time of electric charge accumulation, the higher is the level of the output signals from the electric charge accumulation type photoelectric conversion devices, if the intensity of illumination of the object to be photographed remains constant; while, if the time period for each episode of electric charge accumulation is constant, said output signal level is the higher, the greater is the intensity of illumination of the object to be photographed. Because various types of noise component are inevitably present in the output signals from the electric charge accumulation type photoelectric conversion devices, the lower are the levels of the focus detection signals used for the above described focus detection calculation, the worse are their signal to noise ratios S/N, and this may cause erroneous focusing to occur. Thus, it is necessary to control the length of the time period for electric charge accumulation by the electric charge accumulation type photoelectric conversion devices, so as to make the level of the output signals from the electric charge accumulation type photoelectric conversion devices higher than the level for maintaining a predetermined accuracy of focus detection, in order to obtain high accuracy focus detection. This time period for electric charge accumulation needs to be varied between, for example, about ten microseconds and a few hundreds of milliseconds, presupposing a usual range of intensity of illumination for the object to be photographed.
There are two per se known types of control method for the time period for electric charge accumulation, in order to obtain an accurate result from the focus detection process. In the first of these methods, the electric charge accumulation type photoelectric conversion devices are controlled in the following manner. A target output level for the electric charge accumulation type photoelectric conversion devices is set, and, based upon the maximum value or the average value of the output signals for the current episode of electric charge accumulation and upon the value of the electric charge accumulation time period during this episode, the time period for electric charge accumulation for the next episode of electric charge accumulation is calculated with the aim of bringing the output signal level from the electric charge accumulation type photoelectric conversion devices to equal said target output level. This control method is termed auto gain control (referred to as AGC for brevity hereinafter), because it resembles the process of adjustment of the output level automatically by controlling the amplification gain of the amplifiers used in the electrical circuit. Further, this control process is often termed software AGC, because normally it is performed by a microcomputer. With this software AGC, the method of controlling electric charge accumulation based upon the maximum output level among the outputs of the electric charge accumulation type photoelectric conversion devices is termed peak AGC, while the method of controlling electric charge accumulation based upon the average output level among the outputs of a plurality of electric charge accumulation type photoelectric conversion devices is termed average AGC. In general, peak AGC is widely used at the present.
In order to keep the target output level for peak AGC within the maximum input level of the A/D converter which is being used, even when the intensity of illumination of the object to be photographed varies somewhat, it is typically set to about half of the maximum input level of the A/D converter. If this target output level be termed Dobj, the maximum level among the output signal levels of the electric charge accumulation type photoelectric conversion devices attained for this episode of electric charge accumulation be termed Dmax, and the time period for this episode of electric charge accumulation be termed IT(n), then the time period IT(n+1) for the next episode of electric charge accumulation may be obtained by the following equation: EQU IT(n+1)=IT(n).times.Dobj/Dmax (1)
FIG. 6 is a figure showing the output signal levels for electric charge accumulation type photoelectric conversion devices, in the case that electric charge accumulation time period is controlled according to peak AGC. The electric charge accumulation type photoelectric conversion devices are shown as disposed along the horizontal axis of this figure, while the levels of the output signals of these photoelectric conversion devices are shown along the vertical axis. Although strictly the data for the various photoelectric conversion devices is scattered, nevertheless by the expedient of using this figure a substantially continuous curve can be obtained by connecting together the output levels for the various photoelectric conversion devices.
Another method for controlling the time period for electric charge accumulation by the photoelectric conversion devices is to provide a monitor photodiode neighboring to either one of a pair of A/F sensors, and to terminate electric charge accumulation forcibly when the integrated value of the output signal of said monitor photodiode reaches a previously predetermined standard value. This method is termed hardware AGC, by way of contrast to the software AGC described above.
FIG. 7 is a block diagram showing a prior art type of AGC circuit which performs hardware AGC, while FIG. 8 is a time chart showing the variation of voltage of certain portions of this prior art hardware AGC circuit. In an AF sensor module denoted as 1 and shown as a block there are provided a pair of A/F sensors 1a and 1b and a monitor photodiode 1c which is disposed as neighboring to the A/F sensor 1b. An optical system, not particularly shown, casts an image of the object to be photographed upon the monitor photodiode 1c, said image being almost identical to the image of said object to be photographed which is cast upon the A/F sensor 1b, and accordingly said monitor photodiode 1c outputs a photoelectric current which corresponds to the light intensity of the image of the object to be photographed which is cast upon the A/F sensor 1b. This photoelectric current output by the monitor photodiode 1c is integrated by an amplifier 1d which starts this integration process at the time point of starting of electric charge accumulation by the A/F sensors 1a and 1b, and said amplifier 1d converts said integrated photoelectric current into a voltage signal which it outputs to a comparator 2 as a monitor electrical signal, i.e. as a monitor voltage Vm. This comparator 2 compares the monitor voltage Vm thus generated by the amplifier 1d with a standard voltage Vref, and, as shown in FIG. 8, the output voltage of said comparator 2 remains high level while said monitor voltage Vm remains less than said standard voltage Vref, while said comparator output voltage becomes low level when said monitor voltage Vm attains or becomes greater than said standard voltage Vref. This output of the comparator 2 is supplied to an interrupt terminal INT of a microcomputer 3 which is for controlling electric charge accumulation by the AF sensor module 1, and an interrupt is generated for said microcomputer 3 when said comparator output voltage goes to low level from high level. The interrupt program for the microcomputer 3 which is thus initiated forcibly terminates electric charge accumulation by the A/F sensors 1a and 1b.
The monitor photodiode 1c is disposed adjoining the A/F sensor 1b, and the image of the object to be photographed cast upon said monitor photodiode 1c is almost identical to the image of said object to be photographed which is cast upon the A/F sensor 1b. Therefore, said monitor photodiode 1c outputs a photoelectric current which corresponds to the average amount of electric charge accumulation of the electric charge accumulation type photoelectric conversion devices of said A/F sensor 1b. Accordingly, the monitor voltage Vm output from the amplifier 1d has a voltage value which corresponds to the average value of the focus detection electrical signal which is output from the A/F sensor 1b.
Normally the time period for electric charge accumulation by the A/F sensors 1a and 1b is controlled according to peak AGC. However, because as described above the time period for electric charge accumulation for peak AGC is calculated according to equation (1) when the previous episode of electric charge accumulation has terminated, therefore, if the illumination level of the object to be photographed suddenly increases during the current episode of electric charge accumulation, then it can happen that the level of the electrical signal for focus detection from the A/F sensors 1a and 1b should exceed the maximum input level for the analog to digital converter. In other words, if focus detection calculation is performed based upon this sort of electrical signal for focus detection, the accuracy of focus detection is deteriorated. Therefore, hardware AGC is used as a kind of backup for peak AGC, in consideration of the possibility that the illumination level of the object to be photographed should suddenly change. If in fact the illumination level of the object to be photographed suddenly increases, then the level of the monitor voltage Vm for hardware AGC likewise suddenly increases, and before termination of electric charge accumulation according to peak AGC this monitor voltage Vm reaches the standard value Vref, so that electric charge accumulation by the electric charge accumulation type photoelectric conversion devices are terminated according to hardware AGC.
When the target output level Dobj for hardware AGC and peak AGC is the same, the time period for hardware AGC is set to be longer than the time period for electric charge accumulation calculated for peak AGC. Accordingly, during normal operation, it never happens that during electric charge accumulation according to peak AGC the accumulation of electric charge is forcibly terminated according to hardware AGC. However, when the operator of the camera changes his or her mind as to what object should be photographed so that the level of illumination of the object to be photographed changes abruptly, then it can in fact happen that during electric charge accumulation according to peak AGC the accumulation of electric charge is forcibly terminated according to hardware AGC.
FIG. 9 is a time chart showing such a prior art type of electric charge accumulation action according to both peak AGC and hardware AGC. After electric charge accumulation has been performed for exactly the time period IT(n-1) during the (n-1)th episode of electric charge accumulation according to peak AGC, suppose that the level of illumination of the object to be photographed increases abruptly: then in the next nth episode of electric charge accumulation the monitor voltage Vm produced from the monitor photodiode 1c abruptly increases, and before the time period IT(n) for electric charge accumulation according to peak AGC has elapsed this monitor voltage Vm reaches the standard voltage Vref, and electric charge accumulation is forcibly terminated according to hardware AGC.
Incidentally, because objects to be photographed which have high levels of illumination often also have high contrast, even though it is possible to terminate electric charge accumulation by hardware AGC, at this time it can happen that the levels of the output signals of the A/F sensors 1a and 1b, as shown in FIG. 10, may at some time exceed the maximum input value allowable for the A/D converter. Because it therefore might happen that errors in the calculations for focus detection based upon this type of electrical signal for focus detection might become relatively large, it is necessary to keep the output signals from the A/F sensors 1a and 1b at suitable levels by correcting the time period for electric charge accumulation to be relatively short.
However, because an input signal to an A/D converter whose level exceeds the maximum allowable input level for said converter is treated as having said maximum input level and is analog to digital converted accordingly, the method of calculating the time period IT(n+1) for the next episode of electric charge accumulation according to the previously described equation (1) does not yield a suitable result. Therefore, with prior art type peak AGC, if the signal obtained from the AF sensor module 1 overflows the input range of the A/D converter at any time, then instead of using the equation (1) for calculating the time period for the next episode of electric charge accumulation, the following equation should be used: EQU IT(n+1)=IT(n)/4 (2)
In other words, if the electrical signal for focus detection overflows, then temporarily the time period for electric charge accumulation is compressed on a large scale according to equation (2), and electric charge accumulation is performed accordingly, so that the level of the output signal for focus detection is reliably kept below the maximum input level for the A/D converter, and subsequently calculation according to equation (1) for peak AGC is reverted to.
Nevertheless, with the prior art focus detection devices described above, when the level of illumination of the object to be photographed increases abruptly and at some time the level of the electrical signal for focus detection output by the A/F sensors exceeds the maximum allowable input level for the A/D converter, then, after performing electric charge accumulation with the time period for electric charge accumulation temporarily reduced according to equation (2) in order as shown in FIG. 6 to keep the maximum value of the electrical signal for focus detection less than the target output level Dobj, for several subsequent electric charge accumulation episodes peak AGC is performed according to equation (1). Accordingly there is a problem in the prior art, when the level of illumination of the object to be photographed changes abruptly, with regard to poor responsiveness of focus detection operation.